Touchsmart 300 no boot, fans on loud

[chrisappleby]

This one was brought in by a customer over 2 months ago.
It is a Touchsmart 300-1015uk with an integrated AMD GPU (i.e. no PCI GPU installed).
It will not boot to BIOS, nothing on screen, the fans come on loudly after a few seconds and stay on. No faint inverter image on screen. PSU light stays on. Only other sign of life is a blinking light on the small USB board above the mobo.
So far I've tried reseating CPU and RAM, different RAM, different CPU (which works in other systems), BIOS reset by removing CMOS battery, hard power reset, disconnecting various/all mobo cables. Capacitors look ok.
From reading various repair reports, these systems seem rather trouble prone and parts are hard to source. Parts reliability is therefore a possible issue.
I've so far tried 2 replacement mobos, a replacement PSU and 2 replacement CPUs. In whatever combination I try them I get the same symptoms. The CPUs work ok in other systems. The PSU is a proprietry design but the voltages test ok.
The customer is understandably getting impatient and unless I can resell the parts I've tried so far my profit ill be near zero.
It's turning into the repair job from hell!
Any ideas anyone?

 

[mr m]

Try removing the optical device.

You mentioned what sounds like a USB daughter board sitting atop the mobo. Check it for bent/mangled pins.

Good luck.

 

[Romaniac]

From what I've read on here and around, when systems fans come on and go full blast like that it usually means whatever controls them has stopped controlling them, and is likely dead - in this case, it most likely means the motherboard.

Any searches on the blinking light?

 

[smashedbotatos]

Check for crushed/shorted USB ports.

Another thing I have seen in these is GPU failure.

 

[chrisappleby]

Thanks for the responses guys.
Tried booting already without optical drive and USB daughter board connected, same result.
No bent pins on daughter board either.
Forgot to mention that I tried booting with no RAM and got beeps. So at least the mobo has some life in it.
Googled blinking light on USB board which came up with a few new posts for help but no solutions.
I'm leaning towards a possible previous overheating issue (plenty of anecdotal reports of this in various forums) affecting GPU or caps. Think I might apply some heat to the GPU to see if it comes to life albeit briefly.

 

[JustInspired]

Your 2 replacement mobos... are they supposed to be known good ones?

 

[chrisappleby]

Looks like GPU was to blame. Applied some heat to it, replaced original thermal pad with a homemade copper shim. Applied new thermal paste all round. Now boots ok and temperatures look ok. Will keep monitoring them. All fans running ok too and cleaned them up for good measure.
Thanks to smashedbotatos for the tip.

 

[mr m]

2 replacement mobo's and they all had the same GPU issues? Man, I would have bailed.

Nice job and thanks for the follow up.

 

[chrisappleby]

I was pulling my hair out after the 2nd mobo then the PSU didn't resolve it!
I hate being beaten by these things though. Bloody mindedness at the expense of profitability I suppose.
Best part is that the customer went and bought a new iMac yesterday and now doesn't want to pay full repair price or use the repaired Touchsmart as payment!
What a job it's been!!